Measuring Elapsed Time | Elementary

From 6:15 a.m. to 7:15 a.m., the hour hand makes a full turn around the clock. Therefore, one hour has elapsed.

From 7:15 a.m. to 7:45 a.m. the hour hand moves ahead 30 minutes.

1. I write the subtraction by lining up the hours, minutes, and seconds.

2. I start by subtracting the seconds.

   Since it is impossible to subtract 51 s from 33 s, I regroup 1 min (60 s).
   So, I add 60 s to the 33 s.

   60 s + 33 s = 93 s

   I subtract.

   93 s - 51 s = 42 s

3. Next, I subtract the minutes.

   Since it is impossible to subtract 54 min from 45 min, I regroup 1 h (60 min).
   So, I add 60 min to 45 min.

   60 min + 45 min = 105 min

   I subtract the minutes.

   105 min - 54 min = 51 min

4. Finally, I subtract the hours.
   
   5 h - 4 h = 1 h

1. I write the addition by lining up the hours, minutes, and seconds.

2. I start by adding the seconds.

   59 s + 30 s = 89 s

3. Since there are more than 60 s, I subtract 60 s from 89 s and regroup 1 min.

   89 s - 60 s = 29 s

5. Since there are more than 60 min, I subtract 60 min from 72 min and regroup 1 h.
   
   72 min - 60 min = 12 min

6. Finally, I add up the hours.
   
   3 h + 5 h = 8 h